1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine and a method of manufacturing the same.
2. Description of the Related Art
A spark plug for an internal combustion engine is attached to an internal combustion engine and is used for igniting an air-fuel mixture in a combustion chamber. In general, the spark plug includes an insulating body having an axial hole, a center electrode provided in the axial hole, a metal shell provided on the outer circumference of the insulating body, and a ground electrode which is provided at a front end portion of the metal shell so as to form a spark discharge gap between the center electrode and the ground electrode.
Further, to enhance spark consumption resistance and ignition characteristics, a noble metal tip formed of noble metal alloy such as platinum is bonded to the front end portion of the ground electrode formed of a heat and corrosion resistant metal such as a nickel alloy. When the noble metal tip is bonded to the ground electrode, spot welding by means of a laser beam is performed along the outer peripheral portion of the bonded surface between the ground electrode and the noble metal tip (for example, refer to Japanese Patent No. 3460087).
3. Problems to be Solved by the Invention
Recently, to realize high engine power, there is a tendency to develop high compression ratio engines. In a combustion chamber of such an engine, a noble metal tip or a ground electrode is exposed to high-temperature conditions. In addition, heat is hardly dissipated at the front end side of the ground electrode, and a portion closer to the front end side of the ground electrode is especially subject to high temperatures. Therefore, in the noble metal tip, the ground electrode and the bonded portion therebetween, a difference between heat stress acting on the front end portion of the ground electrode and heat stress acting on the base end portion of the ground electrode may occur. Accordingly, oxidation scales or cracks may develop at the boundary portion between the noble metal tip and the ground electrode, and the noble metal tip may separate from the ground electrode.
Recently, miniaturization of spark plugs has been achieved to accommodate the need for smaller engine size. Further, a metal shell of the spark plug tends to have reduced size and thickness. Therefore, the size of a ground electrode bonded to the metal shell should be reduced, because the bonding area between the metal shell and the ground electrode must be reduced. As a result, the heat dissipation property of the ground electrode is further decreased, and the above-described defects occur more frequently.